1. Field of the Invention
The present invention relates to a pattern forming contrast enhanced material. The present invention also relates to a pattern forming method. More specifically, the present invention relates to a pattern forming method using a contrast enhanced material having such properties that the material has a low primary transmittance to deep ultraviolet ray such as KrF excimer laser and the like, a bleaching action to these rays and an increased transmittance after the perfect bleaching.
2. Description of Related Art
The progress of the high density integration of a semiconductor integrated circuit has been carried out together with a conventional lithography technology and the smallest line width thereof is dashing in a region of submicron. The resolution R of lithography by a reduction projection method is expressed by the following Rayleigh's equation: EQU R=k.sub.1 x .lambda./NA (1)
where
k.sub.1 : process coefficient
.lambda.: wavelength
NA: numerical aperture of lens
From the above equation, a critical resolution is 0.68 .mu.m and insufficient for submicron lithography, when using g-line (0.436 .mu.m) and a stepper of NA of 0.45 and k.sub.1 being 0.7.
Griffing et al. reported that by laminating a contrast enhanced layer for improving the contrast of the optical strength profile on the resist for pattern forming, a process coefficient k.sub.1 can be decreased and a resolution and pattern profile can be improved (B. F. Griffing et al., IEEE ELECTRON DEVICE LETTERS, Vol. EDL-4, No. 1, Jan., pages 14-16 (1983).
According to their report, a resolution of 0.4 .mu.m is possible in the ordinary reduction projecting method (.lambda.: 0.436 .mu.m, NA: 0.32).
Sasago et al. have found that the contrast enhanced material of Griffing et al. requires the formation of a water soluble interlayer thin film on a resist and hence not suitable for practical processes, since it contains an organic solvent, and reported a water soluble contrast enhanced material consisting of a water soluble resin and water soluble photosensitive compound (a diazonium salt) (M. Sasago et al., 1985 SYMPOSIUM ON VLST TECHNOLOGY, Digest of Technical Papers, pages 76-77 (1985).
This water soluble contrast enhanced material is bleached in a wavelength range of 365-406 nm by exposure. Also, since it is water soluble, it can be directly applied on a resist film without being mixed with the resist.
On the other hand, in order to reduce a wavelength of light source, a pattern forming method using KrF excimer laser beam of a wavelength of 248 nm is proposed. The wavelength can be reduced about by 40% by using KrF excimer laser instead of g-line. Therefore considering the above equation (1), it is expected that the resolution can be improved thereby.
Endo et al. reported the use of a compound having ##STR2## group in its molecule as a photosensitive compound in a pattern forming material for deep ultraviolet ray (M. Endo et al., Technical Papers in Regional Technical Conference on Photopolymers Principles Processes and Materials, Oct. 30-Nov. 2, pages 39-50 (1988). Since transmittance relatively greatly changes due to exposure to KrF excimer laser beam in this pattern forming material, it is expected to decrease a process coefficient. However, the change of transmittance is still insufficient for improving contrast and hence not always provide a good result. Also, since the pattern forming material is not water soluble, it is difficult to apply it to practical processes in the same way as the contrast enhanced material of Griffing et al. which requires the formation of a water soluble interlayer thin film.
Endo et al. also reported an excimer laser lithography using a water soluble contrast enhanced material (U.S. patent application Ser. No. 372,227). The contrast enhanced material in this report consists of a photosensitive compound having ##STR3## group in its molecule and a water soluble polymer. A --SO.sub.3 H group attached to an aromatic ring is selected as a water soluble group of the photosensitive compound. However, the --SO.sub.3 H group alone cannot provide an aromatic photosensitive compound having ##STR4## group in its molecule with enough water solubility. Therefore, it is difficult to dissolve the compound in pure water so that the primary transmittance may become sufficiently low and hence a good pattern formation is not expected.
Thus, there has been expected the lithography technology using a water soluble contrast enhanced material capable of decreasing a process coefficient and utilizing deep ultraviolet ray such as KrF excimer laser beam and the like.